This invention relates generally to combustion devices and, more particularly, to emission control systems for combustion devices.
During a typical combustion process within a furnace or boiler, for example, a flow of combustion gas, or flue gas, is produced. Known combustion gases contain combustion products including, but not limited to, carbon, fly ash, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur, chlorine, and/or mercury generated as a result of combusting solid and/or liquid fuels.
The volatile metal mercury, Hg, is an air pollutant produced through coal combustion. Mercury released from coal during combustion is readily aerosolized and can become airborne. Airborne mercury may travel globally prior to being deposited into soil and water. Mercury deposited in the environment is a persistent and toxic pollutant that may accumulate in the food chain. For example, mercury can be transformed within microorganisms into methylmercury, and consumption of contaminated fish is the major route of human exposure to methylmercury. Methylmercury is toxic to humans and has been shown to cause disorders of the nervous system, comas, heart disease, and death. Moreover, the adverse affects of methylmercury may be more severe to children and women of childbearing age.
Mercury emissions from coal-fired power plants are the subject of governmental regulation. The control of mercury emissions is complicated by the several forms mercury may take within combustion flue gas. For example, at combustion temperatures, mercury is present in flue gas in its elemental form, Hg0, which may be difficult to control because elemental mercury is easily volatized and unreactive. Mercury reacts with carbon as flue gas cools below 1000° F., and such reactions may convert mercury to its highly reactive, oxidized form, Hg+2. Mercury may also be absorbed in fly ash and/or other flue gas particles to form particulate-bound mercury, Hgp.
Because mercury can take several forms, known control technologies do not effectively control mercury emission for all coal types and for all combustion configurations. Some known mercury control technologies take advantage of mercury's reactivity with carbon and use carbon as a mercury sorbent to form oxidized mercury. Carbon may be formed in-situ during the combustion process as a result of incomplete coal combustion or may be injected into mercury-containing flue gas in the form of activated carbon. Further, carbon in the presence of chlorine, Cl, may increase the oxidation of elemental mercury. In the flue gas, mercury can be converted to its oxidized form, Hg+2, and react with chlorine-containing species to form mercuric chloride, HgCl2. As such, the extent of mercury oxidation in flue gas is generally higher for coals with a higher chlorine content, such as bituminous coals, and lower for coals with a lower chlorine content, such as low-rank coals.
Mercury may be removed from flue gas by reacting with carbon in high-carbon fly ash formed in-situ in the combustion process. High-carbon fly ash is formed during the combustion of bituminous coals in coal reburning and air staging, and may be an effective mercury sorbent. Other coals, such as, for example, Powder River Basin (PRB) and lignite coals, are considered low-rank coals, and as such, represent a significant portion of the coal energy market. Such coals often have a low sulfur content that solves the problem of sulfur dioxide, SO2, emissions, but may also have a low chlorine content. As such, the mercury in low-rank coals may not be oxidized because of a lack of chlorine and the presence of other constituents that tend to suppress mercury oxidation. As a result, mercury released during combustion is primarily elemental mercury. Moreover, because of the high reactivity of low-rank coals, fly ash from the combustion of such coals usually has a low carbon content. Coal reburning and air staging, which typically increases the carbon content in fly ash for bituminous coals, usually does not significantly increase the carbon-in-fly ash content for low-rank coals. As such, mercury removal through reactions with carbon-in-fly ash may not be effective because such fly ash does not have a sufficient amount of carbon with which the mercury can react.
One known mercury control technology injects a sorbent, usually activated carbon, into the flow of flue gas to react with mercury therein. Because carbon is more reactive towards mercury at temperatures below 350° F., activated carbon is typically injected upstream from a particulate collection device, such as an electrostatic precipitator or a baghouse. Oxidized mercury is the most easily removable form of mercury by injecting sorbent. As a result, the higher the fraction of oxidized mercury in flue gas, the higher the efficiency of mercury removal. Depending on the sorbent injection configuration and coal type, the efficiency of mercury removal typically ranges from 40% to 90% removal of mercury emissions. However, the cost of using activated carbon for mercury control may be expensive, and as such, mercury emission control may be affected by cost of the sorbent.
Efficiencies of most available mercury emission control technologies depend on the mercury speciation in flue gas. Oxidized mercury is water-soluble and may be removed from flue gas using known wet desulfurization systems (wet-scrubbers). At least some particulate-bound mercury may be removed from flue gas using known particulate collection systems. Elemental mercury is more difficult to remove than oxidized mercury and/or particulate-bound mercury because elemental mercury is unreactive and, as such, cannot be removed from flue gas with wet desulfurization systems or particulate collection system.
In some known systems, because the concentration of mercury in the flue gas is very small (typically less than 10 parts per billion or ppb), diffusion of mercury from the surrounding flue gases may limit the mercury removal process. Most of the flue gases produced in known systems flows in substantially laminar flow patterns and is characterized by slow diffusion rates. Because of the flow characteristics of the flue gas, some known mercury emission reduction systems have attempted to optimize the use of the sorbent by modifying the number and design of sorbent injection lances to achieve sorbent coverage within the flue duct.